Learn Portuguese (Portugal): Ambos os machos eram do mesmo dono.

In Portuguese, the quantifier ambos (both) requires a definite article when it directly precedes a noun. While English just says "both males", Portuguese requires the literal structure "both the males" (ambos os machos). If the noun were feminine, you would use ambas as.

Why use the imperfect tense eram here instead of the preterite foram?

We use the imperfect (eram) because this describes a continuous state or background context in the past. Being owned by someone is an ongoing situation, not a single, completed action with a clear start and end point, which is what the preterite (foram) would imply.

Why is it do mesmo dono instead of just de mesmo dono?

The word mesmo (same) usually requires a definite article before it. So, "from the same owner" translates as de (of/from) + o (the) + mesmo dono (same owner). The preposition de and the article o contract to form do.

Could machos refer to human men in this sentence?